Non-initial empty documents (about:blank) should inherit their cross-origin-opener-policy from the navigation's initiator top level document, if the initiator and its top level document are same-origin, or default (unsafe-none) otherwise.

1. Create the opener popup with a given COOP openerCOOP.
2. Add iframe to the opener popup that is either same-origin or cross-origin.
3. Opener's iframe opens a new window, to a network document with openeeCOOP.
4. Opener's iframe navigates the openee popup to about:blank.